The present invention relates generally to methods for laser cutting a hollow workpiece, such as a length of tubing. The present invention is more particularly directed to methods for fabricating medical devices, such as, for example, expandable endoprostheses, commonly known as stents, using a sacrificial masking material which helps to prevent damage to the workpiece when cutting the workpiece with a laser apparatus that utilizes an inert gas, instead of air or oxygen, in the cutting process.
Stents are particularly useful in the treatment and repair of blood vessels after a stenosis has been compressed by percutaneous transluminal coronary angioplasty (PTCA), percutaneous transluminal angioplasty (PTA), or removed by atherectomy or other means, to help improve the outcome of the procedure and reduce the possibility of restenosis.
Stents are generally cylindrically shaped devices which function to hold open, and sometimes expand, a segment of a blood vessel or other arterial lumen, such as a coronary artery. Stents are usually delivered in a compressed condition to the target site and then deployed at that location into an expanded condition to support the vessel and help maintain it in an open position.
Prior art stents typically fall into two general categories of construction. The first type of stent is expandable upon application of a controlled force, often through the inflation of the balloon portion of a dilatation catheter which, upon inflation of the balloon or other expansion means, expands the compressed stent to a larger diameter to be left in place within the artery at the target site. The second type of stent is a self expanding stent formed from shape memory metals or super elastic nickel titanium alloys (Nitinol), which will automatically expand from a compressed state when the stent is advanced out of the distal end of the delivery catheter into the blood vessel.
Stents can be formed with strut patterns which when expanded have a large amount of open space, but when collapsed have little space between the often tortuously shaped struts forming the stent. One method of making a stent includes laser cutting a tubular member or tubing of suitable material to create the intricate strut patterns which define the structure of the stent. Laser cutting generally provides a precise method for forming these intricate strut patterns in the tubing used to form the stent. Such patterns require the tubing to be cut through one side of the wall of the tubing without cutting through the opposite side of the tubing.
In the past, laser apparatus utilizing pressured air (oxygen) have been used to cut the tubing. Generally, a laser beam locally heats the tubing material while pressurized air is blown through a small coaxial orifice directly onto the heated region in order to create a slot or “kerf” through the wall of the tubing.
Laser cutting of a length of tubing generally begins by focusing a laser beam on a targeted spot on the tubing. The spot is melted and is preferably vaporized, or at least partially vaporized, by the laser beam. Once the laser beam burns through the side wall of the tubing, the beam will usually continue to strike the opposite side wall of the tubing, and may begin to vaporize, or partially vaporize, the opposite side wall of the tubing. This undesirable burning or partial vaporization of the opposite sidewall is called “burn through” and can result in the weakening of opposite sidewall. In some cases, burn through may result in the workpiece being discarded. The melting and vaporization of the tubing also can form “recast” material, which is material from the tubing that has melted and resolidified on laser-cut surfaces. The recast material, also referred to as “dross,” may include metal oxides and impurities which are undesirable in the manufacturing process since the recast material must be thoroughly removed from the surface of the stent. Oxidation can make a stent more susceptible to failure (e.g., cracking or fracture) during manufacturing or, if not completely removed, in use. Additionally, recast material can be particularly difficult to remove without damaging the thin struts created by the laser cutting operation. Therefore, both burn through and formation of recast material presents a formidable problem to the stent manufacturer.
The problems of laser cutting self-expanding stents made from a material such as Nitinol are further enhanced when pressurized air or oxygen is used to create the cut pattern. Because Nitinol is composed of about 50% titanium, a notoriously reactive metal, the titanium readily reacts with the oxygen in the air when heated. As a result, the material expelled during the cutting procedure is predominately comprised of metal oxides, most of which are trapped inside the tubing and adhere to the metallic inner surface of the Nitinol tube. Side walls of the slot or kerf also become oxidized during the cutting process, making the as-cut stent less ductile and thereby more susceptible to cracking or complete fracture during radial expansion or during other subsequent manufacturing steps. As a result, a laser cut Nitinol work piece must be carefully processed by a number of different cycles of chemical treatment, radial expansion, and heat stabilization to achieve the final stent size.
Any remaining oxidized wall material and other adhered oxide debris must first be removed in order to attain an acceptably smooth surface later during electropolishing. This additional clean up procedure can be achieved through a combination of automated grit blasting, manual grit blasting and chemical removal of oxide prior to electropolishing. Some methods require the physical removal of the recast material using a reamer or similar equipment and can often damage the thin struts of the stent. While electropolishing procedures can remove some recast material, often the recast material may be so heavily clad on the surface of the stent that not all of the recast material can be removed by this process. Additionally, the electropolishing process will remove material from the struts so it is important that only a small amount of the strut surface is actually removed. For example, if the electropolishing procedure is too long in duration, due to accumulated recast material, portions forming the struts of the stent may have too much material removed, resulting in a damaged or generally weakened stent.
Certain methods have developed to deal with the problem caused by burn through and the formation of recast material on the workpiece. One such method uses a continuous metal wire run through the tubular workpiece to create a “protective barrier” which somewhat helps to prevent the laser beam from striking the opposite sidewall of the tubing. Another system utilizes a liquid flushed through the workpiece as it is being cut. The fluid is usually fed through one end of the tubing and exits through the opposite end of the tubing, along with the newly formed openings in the wall of the tubing created by the laser. The liquid flushes away some of the recast material being created by the vaporization of the tubing. However, the presence of this liquid does not always completely block the laser beam, which can allow the inside wall of the tubing to be heated and damaged. Additionally, the use of liquid requires additional equipment for handling the liquid including discharge equipment, catch basins, waste disposal, and other processing equipment.
It has been anticipated that Nitinol stents could be laser cut using an inert gas such as argon or helium to prevent sidewall oxidation which would help prevent cracking or fracturing during subsequent processing. The absence of oxygen in the cutting process also will help to prevent the recast material from being oxidized. However, laser cutting Nitinol tubing utilizing pressurized argon gas typically cannot directly produce a finished stent because the expelled melted material formed during the cutting process can become “welded” to the inner wall of the tubing. This welded metallic build up could possibly be removed by later processing including reaming, drilling, electric discharge machining and the like but with difficulty and risk to the integrity of the workpiece.
What have been needed and heretofore unavailable are improved methods for reducing the adverse results caused by burn through along with the elimination of oxidation during the laser cutting process. The present inventions disclosed herein satisfy these and other needs.